Russian President Vladimir Putin gives the separatists "exactly what they were short of," said Stephen Blank, an analyst at the American Foreign Policy Institute.

By contrast, the Obama administration has rejected similar support to Ukraine's national government, saying its goal is to de-escalate tensions and achieve a broader peace, not further inflame the conflict.

That policy is being tested by Russia's military role in the separatists' campaign to create an autonomous state of largely ethnic Russians in eastern Ukraine, near Russia's border.

The Pentagon says Russia's military fires at Ukrainian forces from inside Russia, and the Russians are moving heavier rockets toward the border that could be handed over to the separatists.

"We have indications that the Russians intend to supply heavier and more sophisticated multiple-launch rocket systems in the very near future," Col. Steve Warren said Friday. "We do believe it is imminent."

The larger-caliber rockets are typically used against ground targets and have ranges of more than 10 miles.

The rockets can be fired at troop formations, air fields, staging areas, artillery positions and other "area targets," said Jim Howcroft, a retired Marine Corps intelligence officer with extensive experience in the region. That capability would allow rebels to break up offensives launched by Ukrainian forces.

The Russians have provided rebels with anti-aircraft systems. The Pentagon said a surface-to-air missile probably provided by Russia and fired from rebel-held territory downed Malaysia Airlines Flight 17 last week, killing all 298 people aboard.

Wednesday, two Ukrainian attack jets were shot down over rebel-held territory, the latest in a string of Ukrainian military aircraft downed in the past several months.

Typically, conventional forces enjoy an advantage over rebels through their use of aircraft and firepower. Russian support helps to neutralize that edge, analysts said.

"The poor Ukrainians now have two fights on their hand," Howcroft said. They face an irregular army supported by firepower and anti-air weapons associated with conventional forces, he said.

The boost in Russian support comes in the wake of successes Ukrainian forces had in reclaiming some key areas from rebel control. The Russians have massed about 12,000 troops along the border with Ukraine, according to the Pentagon.

The international condemnation aimed at Russia after the downing of Flight 17 has not deterred Russia' from arming the rebels, according to U.S. officials. "There has been a continuous flow over the last several weeks of weapons and equipment from Russia to Ukraine," Pentagon spokesman Warren said.

The Ukrainian government in Kiev has asked for more potent weapons, but the United States is sticking to its policy of providing only non-lethal aid.

A Pentagon team visited Ukraine in June and another one is due to visit in a few weeks, the Pentagon said. That raises the possibility that the administration will reverse course.

"We have a very active, ongoing process to think through what support we may provide to ... Ukraine," Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, said Thursday. "That debate is ongoing."

So long as U.S. aid remains limited and Russia can supply weapons easily across a long border, Ukraine's national armed forces will have difficulty defeating the rebels, Howcroft said. "There's no way the Ukrainians can stamp this out while it has a porous border with Russia," he said.